Homing guided missiles, particularly those launched against airborne targets, may have a range denied mode of operation. An example of a range denied mode is that of attacking an airplane with a very powerful jammer. While the self-protect jammer makes it easy to determine the angle from the missile to the target, it makes it impractical if not impossible to obtain and interpret a radar ranging return. Thus, the attacking aircraft has no information about the range to the target. Or, if tracking with an optical seeker, angle information may be obtained to substantial accuracy, but ranging with optical data is difficult, especially if the target optical size is unknown. In the range denied mode of operation, the missile may be required to function without range information in the "end game," resulting in reduced effectiveness of the missile. The effectiveness of a missile is not determined by whether it comes to the general vicinity of the target, but by how small a distance it misses the target. The last few seconds or even milliseconds of the engagement, when miss is really being discovered/determined, are termed the "end game."
Conventional missile guidance systems operating in the range denied mode have used proportional navigation in the end game, and suffered the near miss degradation due to the settling time following transients that occur near to zero range to the target. This has led to the rule of thumb that target acquisition must occur at about ten "time-constants to go," i.e. at ten time-constants prior to intercept. It is common to use first order feedback system terminology in a discussion of the end game. A time-constant is the time it takes, following a disturbance, to reduce the error induced to about 2/3 of its value. The state of the art has reduced the time-constant of a missile to the 100 millisecond or less domain. It has been discovered that acquisition occurrence can be reduced to two or three time-constants to go if range data is available to the guidance law. Thus there is a need for a range or time to go estimate.
Conventional angle only ranging schemes have required very large maneuvers to make the range observable. These large maneuvers were needed because the range estimators did not take advantage of the inherent geometrical dynamics associated with a homing missile's approach to the target. The conventional angle only ranging schemes have not made use of the six correlations available, and the sensitivity of the guidance loop stability margin to the range estimate in a range dependent guidance law, a feature in modern guidance laws that adds to the observability of range from angle only information. Moreover, such conventional schemes have not made use of correlation of noisy signals to classify in real time the usefulness of each of the six potentially available range error signals.